1. Field of the Invention
The present invention relates to an air-fuel ratio control system for an internal combustion engine, and more specifically to the so called air intake side secondary air supply system for an internal combustion engine.
2. Description of Background Information
In an internal combustion engine provided with a three-way catalytic converter in the exhaust system, the air-fuel ratio of the mixture supplied to the engine is controlled around a stoichometric value (14.7:1 for example) by a feedback control which is performed according to the composition of the exhaust gas and operating conditions of the engine. This is because an optimum operation of the three-way catalytic converter is enabled at the stoichometric air-fuel ratio. The air intake side secondary air supply system for the feedback control is an example of this type of feedback control system and which is constructed such that an air intake side secondary air supply passage leading to downstream side of the throttle valve is provided. The air fuel ratio control is performed by varying the amount of the secondary air flowing through the air intake side secondary air supply passage.
In the air intake side secondary air supply system, it is general to stop the supply of the air intake side secondary air during an engine warm-up period after a cold start of the engine, so that the air-fuel ratio feedback control is stopped and a rich air-fuel mixture is supplied to the engine. This is because the combustion condition of the engine tends to be unstable during such an engine warm-up period. The feedback control of the air-fuel ratio starts when the warming-up of the engine is completed. Since the air-fuel ratio of the mixture supplied to the engine varies depending on the intake air temperature, it is suitable to determine the timing of the start of the air-fuel ratio feedback control using the intake air temperature. Therefore, the supply of the air intake side secondary air is stopped to enrich the air-fuel mixture when the intake air temperature is lower than a predetermined level (18.degree. C. for example).
Further, in a system described in Japanese Utility Model Application No. 58-134919 which is assigned to the same assignee of the present application, the feed-back control of the air-fuel ratio is stopped to enrich the air-fuel ratio when the engine coolant temperature is below a predetermined temperature level T.sub.2 (70.degree. C. for example) and the vehicle speed is lower than a predetermined speed V.sub.1 (15 Mile/h) even though the intake air temperature is above the predetermined level T.sub.1. The stopping of the air-fuel ratio feedback control during the cold engine period is determined in consideration of the closure of a choke valve of the engine and of the relatively small amount of the main intake air. Specifically, if the supply and the stop of the air intake side secondary air are repeated when the choke valve is closed so as to enrich the air-fuel ratio during the cold engine period, that will prevent the necessary enrichment of the air-fuel mixture. Moreover, since the proportion of the amount of the air intake side secondary air relative to the amount of the main intake air is high when the vehicle speed is low, the variation of the amount of the air intake side secondary air as a result of the supply and stop thereof is likely to cause hunting of the engine rotation. Thus, the driveability of the vehicle will be deteriorated if the air intake side secondary air is supplied during a period in which the engine is cold.
In this air intake side secondary air supply system, the feedback control of the air-fuel ratio with the air intake side secondary air is executed when the intake air temperature is above a predetermined temperature level T.sub.1 and at the same time the vehicle speed is above the predetermined speed V.sub.1, even through the engine coolant temperature is below the predetermined temperature level T.sub.2. In other words, the feedback control of the air-fuel ratio is executed preferentially as the vehicle speed is high enough to avoid the hunting of the engine rotation. Specifically, under this condition, the amount of the main intake air is relatively large and the repetition of the supply and stop of the air intake side secondary air scarcely cause the hunting of the engine rotation even through the choke valve is not completely open. However, if the clutch of the vehicle is operated to disconnect the driving power of the engine under this condition, the supply and stop of the air intake side secondary air causes the hunting of the engine rotation since no load is applied to the engine. Thus, the driveability of the vehicle is deteriorated with the conventional system construction.